Conventionally, a solvent deasphalting (SDA) process is employed by an oil refinery for the purpose of extracting valuable components from a residual oil feedstock, which is a heavy hydrocarbon produced as a by-product of refining crude oil. The extracted components are fed back to the refinery wherein they are converted into valuable lighter fractions such as gasoline. Suitable residual oil feedstocks which may be used in a SDA process include, for example, atmospheric tower bottoms, vacuum tower bottoms, crude oil, topped crude oils, coal oil extract, shale oils, and oils recovered from tar sands.
In a typical SDA process, a light hydrocarbon solvent is added to the residual oil feed from a refinery and is processed in what can be termed as an asphaltene separator. Common solvents used are methane, ethane, propane, butane, isobutane, pentane, isopentane, neopentane, hexane, isohexane, heptane, their mono-olefinic counterparts thereof, and similar known solvents used in deasphalting, and mixtures thereof. Under elevated temperature and pressures, the mixture in the asphaltene separator separates into a plurality of liquid streams, typically, a substantially asphaltene-free stream of deasphalted oil (DAO), resins and solvent, and a mixture of asphaltene and solvent within which some DAO may be dissolved. The SDA process is a well-known petroleum process and is described in U.S. Pat. No. 3,968,023 to Yan, U.S. Pat. No. 4,017,383 to Beavon, U.S. Pat. No. 4,125,458 to Bushnell, all incorporated herein by reference, and numerous others.
Once the asphaltenes have been removed, the substantially asphaltene-free stream of DAO, resins and solvent is normally subjected to a solvent recovery system. The solvent recovery system of an SDA unit extracts a fraction of the solvent from the solvent rich DAO by boiling off the solvent, commonly using steam or hot oil from fired heaters. The vaporized solvent is then condensed and recycled back for use in the SDA unit.
Often it becomes beneficial to separate a resin product from the DAO/resin product stream. This is normally done before the solvent is removed from the DAO. xe2x80x9cResinsxe2x80x9d as used herein, means resins that have been separated and obtained from a SDA unit. Resins are denser or heavier than deasphalted oil, but lighter than the aforementioned asphaltenes. The resin product usually comprises more aromatic hydrocarbons with highly aliphatic substituted side chains, and can also comprise metals, such as nickel and vanadium. Generally, the resins comprise the material from which asphaltenes and DAO have been removed.
U.S. Pat. No. 3,775,292 to Watkins teaches a SDA process where feedstock is deasphalted using a solvent, and then the resin is removed using a selective solvent in a solvent extraction unit so as to provide solvent-lean resin concentrate and a de-resined second liquid phase. Neither solvent is recovered, as the resin and the DAO is further processed in a hydrocracking unit so as to produce lower boiling hydrocarbons.
U.S. Pat. No. 4,101,415 to Crowley and U.S. Pat. No. 4,686,028 to Ven Driesen et al. teach similar SDA processes where a feedstock is subjected to a solvent extraction step that removes both the asphaltenes and the resin, resulting in an asphaltene-free and resin-free DAO. The asphaltene/resin mixture removed from the feedstock is then subjected to a second solvent extraction step that separates the resins from the asphaltenes.
U.S. Pat. No. 4,239,616 to Gearhart teaches a SKA process where a heavy hydrocarbon material is mixed with a solvent and then subjected to a first separation zone at elevated temperature and pressure so as to effect a separation of the material into a first light phase comprising oils, resins, and solvent, and a first heavy phase comprising asphaltenes and some solvent. The first light phase is sent to a second separation zone where it is subjected to temperatures higher than those in the first separation zone so as to effect a separation of the of the first light phase into a second light phase comprising oil and solvent and a second heavy phase comprising resins and some solvent. The second light phase is then sent to a third separation zone where it is separated into a third light phase comprising solvent and a third heavy phase comprising oils.
A key element of the ""616 process is that at least a portion of the first heavy phase is introduced into the upper portion of the second separation zone. This is done so as to contact the first heavy phase with the second light phase and remove at least a portion of any resinous bodies that may be entrained in the second light phase. It is also preferred that the first heavy phase is sufficiently heated prior to its introduction into the second separation zone so as to cause the formation of internal reflux within the upper portion of the second separation zone, thus assisting in resin removal from the second light phase.
U.S. Pat. No. 4,454,023 to Lutz teaches a process whereby a heavy viscous hydrocarbon feed is processed in a visbreaker unit and fed to a distillation unit for fractionation. The bottoms product of the distillation unit is then fed to a solvent extraction unit producing a heavy asphaltene fraction as well as one or more lighter fractions which contain a large percentage of resins or oils. At least a portion of the lighter fractions that contains resin is recycled back to the feed stream to the visbreaker so as to increase the conversion in the visbreaker.
U.S. Pat. No. 5,145,574 to Hedrick teaches a process separating a resin phase from a solvent solution containing a solvent, DAO, and resin. The solvent solution is introduced into a special heat-exchange apparatus and directed over at least a portion of a generally vertically positioned heat-exchange surface thereby heating the solvent solution to precipitate the resin phase. A solvent solution having a reduced resin content is then recovered, as well as a resin product.
A separate, deasphalted resin product makes a better feed for heavy hydrocarbon cracking units such as H-OIL(trademark), delayed cokers, and visbreaker units. Resin-free DAO is also an improved feedstock for product cracking units such as hydrotreaters, hydrocrackers, and catalytic cracking units.
H-OIL(trademark) is a proprietary ebullated bed process (co-owned by Hydrocarbon Research, Inc. and Texaco Development Corporation) for the catalytic hydrogenation of heavy vacuum residuum, or xe2x80x9cresid,xe2x80x9d and heavy oils to produce upgraded distillate petroleum products and an unconverted bottoms product particularly suited for blending to a low sulfur fuel oil. In the H-OIL(trademark) process, a catalyst is contacted with hydrogen and a sulfur- and metal-containing hydrocarbon feedstock by means which insures that the catalyst is maintained at essentially isothermal conditions and exposed to a uniform quality of feed. This hydroprocessing process is particularly effective in achieving high levels of hydrodesulfurization with vacuum resid feedstocks. The H-OIL(trademark) product is characterized as a liquid product of lower density and average boiling point, lower sulfur content, and lower content of metals.
High conversion is difficult at times because resid feedstocks typically contain high concentrations of metals such as nickel, iron and vanadium as well as high concentrations of nitrogen and sulfur. Many of these materials can even deactivate or poison catalysts. Poisoning of the catalyst often leads to the need for frequent catalyst additions or changeouts which impact unit availability and throughput. Resid conversion also is difficult because resid feedstocks contain a large asphaltene fraction that produces insoluble carbonaceous material when the feedstock is heated. Formation of these solids often results in feedstock or temperature operating limitations. Resin feedstocks are lower in metals content and asphaltenes, and are thus better feedstocks for the H-OIL(trademark) process than resids.
The delayed coking process is an established petroleum refinery process which is used on very heavy low value resid feeds, such as vacuum residue, to obtain lower boiling cracked products. In the delayed coking process, the heavy oil feedstock is heated rapidly in a tubular furnace from which it flows directly to a large coking drum which is maintained under conditions at which coking occurs under a slight pressure. In the drum, the heated feed decomposes to form coke, gas and desired lower boiling liquids which are removed from the top of the drum and passed to a fractionator. When the coke drum is full of solid coke, the feed is switched to another drum and the full drum is cooled and emptied of the coke product. Generally, at least two coking drums are used so that one drum is being charged while coke is being removed from the other.
Coking in the furnace is a significant problem in delayed coking operations. Higher temperatures in the coking drums can reduce the yields of coke and gas. The furnace provides the higher temperatures, leading to excessive fouling in the furnace tubes, thus leading to a greater maintenance requirement to clean the furnace tubes.
Various modifications have been made in the basic delayed coking process. For example, U.S. Pat. No. 4,455,219 to Janssen et al, and U.S. Pat. No. 4,518,487 to Graf et al, both incorporated herein by reference, provides modifications to the delayed coking process by replacing some of the heavy feed with lower boiling range hydrocarbons. These procedures result in improved coking processes in which increased liquid products are obtained with a corresponding reduction in coke yield. Thus, resin feedstocks are better feeds for a delayed coking unit than resids because of the lighter nature of the resins, as well as their propensity to reduce coking in the furnace.
Visbreaking (a term of art used as an abbreviation for xe2x80x9cviscosity reductionxe2x80x9d) is a mild cracking operation used to reduce the viscosity of resid by converting the resid to lighter hydrocarbon fractions. The resid is sometimes blended with valuable light oil, or cutter stocks, to produce oils of acceptable viscosity. By the use of visbreakers, the viscosity of the resid is reduced so as to lower the requirement of the cutter stock. To improve process economics, visbreaking is generally preformed at high temperatures and pressures so as to increase conversion of the heavy residue. Depending on the severity of the visbreaking operation, coking and fouling of equipment may occur during the visbreaking reaction, which limits the ability to increase the severity of the visbreaking operation. Thus, for a given feedstock, the greatest conversion could be achieved by increasing severity; however, such increase in severity may adversely affect product quality and/or the rate of coke formation, whereby the ability to increase conversion by increasing severity is limited. Examples of a visbreaking process can be found in U.S. Pat. No. 5,925,236 to Fersing, et al., U.S. Pat. No. 5,413,702 to Yan, both incorporated herein by reference, and numerous others.
Various schemes have been proposed for increasing the severity of a visbreaking operation. For example, U.S. Pat. No. 4,454,023 to Lutz, incorporated herein by reference, proposes to increase the severity of a visbreaking operation by subjecting heavy product from the operation to a solvent extraction step to produce, as separate fractions, solvent extracted oil, resin and asphaltene, with the resin fraction being recycled to a visbreaking operation to permit an increase in severity. In general, resins are preferred feedstocks over asphaltenes for these types of units due to their reduced viscosity, lower solids content, and because they allow a visbreaking operation to run at more severe conditions. U.S. Pat. No. 4,767,521 to Feldman, et al., incorporated herein by reference, proposed to increase the severity of visbreaking by removing some heavy components from the visbreaker feed.
The hydrocracking unit is the most versatile of refinery conversion units. It can process a wide range of feedstocks from naphtha to asphalt to yield any desired product with a molecular weight lower than that of the feedstock. Hydrotreating is the most widely used catalytic refinery process and can treat feedstocks from the lightest naphthas to the heaviest vacuum resids. It is used primarily to remove undesired impurities, such as sulfur containing compounds, from the feedstocks. Both hydrocracking and hydrotreating utilize hydrogen as a reactant. Catalytic cracking is similar to hydrocracking, except that no hydrogen is used. In each process, a catalyst is used which can become deactivated by any metal or solid impurities found in the feedstock, as well as by any coke produced in the process.
Solid impurities also cause poor flow patterns in the reactors, as well as fouling, plugging, and blocking of conduits and downstream equipment. Oils laden with solids cannot be efficiently or readily pipelined. Buildup of solids can lead to equipment repair, shutdown, extended downtime, reduced process yield, decreased efficiency, and undesired coke formation. A resin-free DAO feedstock is, thus, preferred to a resin-containing DAO feedstock because, as stated above, in removing the resin portion of the DAO most of the metals, solids that remain after asphaltene removal and coke producers are also removed from the DAO.
One aspect of this invention provides an improved process for separating a resin phase from a solvent solution comprising a solvent, deasphalted oil (DAO) and resin. This improved process comprises heating the solvent solution so as to precipitate the resin from the solvent solution, and then separating the resin and some solvent from the solvent solution. This will produce a resin product and a mixture comprising the DAO and the remaining solvent. The DAO/solvent mixture is then boiled so as to vaporize a fraction of the solvent. The vaporized solvent is removed from the DAO/solvent mixture leaving a resin-free DAO product that contains any unvaporized solvent. The vaporized solvent is then condensed and recycled back to a solvent deasphalting phase to be reused in deasphalting a heavy hydrocarbon feedstock.
More specifically, this invention involves heat integration of a solvent deasphalting process with a gasification process. The heat integration involves boiling the mixture of DAO and solvent using waste heat from a gasification unit, preferably heat from hot, saturated syngas. Resin removal is accomplished by heating the aforementioned solvent solution with the vaporized solvent fraction. The DAO/solvent mixture is also preheated with the vaporized solvent fraction prior to boiling. The solvent solution heating and DAO/solvent mixture preheating steps are usually done in series, where the DAO/solvent mixture is first preheated prior to the boiling step leaving a cooled vaporized solvent fraction. The cooled vaporized solvent fraction is then used to provide the heat to the solvent solution prior to the resin separation step. The asphalt that is recovered in the deasphalting step is then used as feedstock in the gasification process.